Non-editable Thank You Page URL

In previous versions of the plugin I was able to edit the plugin file “seamless-donations/dgx-donate-paypalstd.php” to return the Thank You page “success URL” that we setup in PayPal and on WP ( http://almanacofdemocracy.org/donate/thankyou/) rather than what’s initiated by the plugin as it was returning a 404 page.

Now this file is the only one that is not editable, can someone let me know why I can’t edit it or send a possible work-around so that I can setup the return “Thank You” page URL to the page we have created?

Can you provide more detailed info on how to configure the Thank you page? I see the Thank you Page text in the Seamless Donations submenu and the Thank you email is working fine, but do I need to create a new WordPress page for this or do I have to configure it in Paypal in order for it to work?
The site isn’t launched at the primary domain as yet, but we’re almost ready.

After donations are received, I’d like to push the user to the thank you page that I’ve already set up on my WP site, rather than the one in the plugin. Can you provide directions on how I might be able to do this? Thanks!